As society develops, the problem of environmental pollution has become a factor restricting rapid economic growth. Therefore, environmental protection is essential for government legislation in countries around the world. Industrially developed countries have very strict requirements on sulfur emissions. The United States Environmental Protection Agency specifies through laws and regulations, an upper limit for the concentration of SO2 as 50 ppm (v), equaling about 143 mg/m3, in flue gases emitted from heating furnaces of the petroleum refining industry, in sulfur-containing tail gases, and in flue gases regenerated during catalytic cracking.
Currently, the Integrated Emission Standard of Air Pollutants (GB16297-1996) is being implemented in China for controlling the concentration of SO2 in the flue gases emitted from a sulfur plant. This Standard prescribes that the concentration of SO2 in emissions should be lower than 960 mg/m3. In a new environmental protection standard to be implemented, it stipulates an upper limit for the concentration of SO2 in the flue gases emitted from a sulfur plant as 400 mg/m3, and a particular upper limit thereof as 200 mg/m3. However, the concentration of SO2 in the flue gases emitted from most sulfur plants is only below 960 mg/m3 at present, with the standard requirement of 200 mg/m3 hard to achieve.
Liquid sulfur degassing is an important measure for safe production in a sulfur recovery plant. The concentration of H2S in sulfur produced by a Claus process usually ranges from 300 to 500 ppm. Without liquid sulfur degassing, H2SX dissolved in the liquid sulfur will decompose into H2S, which will be released during storage, transportation, and processing steps of H2SX. When H2S accumulates to a certain concentration, such risks as toxicity and even explosion will be generated. In addition, solid sulfur molded from non-degasified sulfur will be subject to high fragility, and thus more sulfur granule and dust will be generated in loading, unloading, and transportation of the solid sulfur.
JACOBS employs Shell Group of Companies' patented technology of liquid sulfur degassing, wherein a stripping tower is provided in a liquid sulfur tank. Air is fed into the stripping tower, and under intense agitation of the air flow, H2SX dissolved in the liquid sulfur decomposes into H2S, which enters a gas phase space along with the air. The gas after being removed of H2S is pumped into a tail gas incinerator by a steam ejector. KTI uses BP Amoco's patented technology of liquid sulfur degassing. In this technology, a stripping tower in the form of a packed tower for catalytic reactions is arranged above a liquid sulfur tank. Liquid sulfur contained in the liquid sulfur tank, with the pressure thereof being elevated through a pump, enters the stripping tower from therebelow along with the air. After the liquid sulfur and the air pass through a packing layer of the packed tower, H2SX dissolved therein decomposes into H2S, which enters a gas phase space along with the air to form a gas mixture. Subsequently, the gas mixture is fed into an incinerator. A liquid sulfur degassing procedure developed by Siirtec Nigi (SINI) adopts a sieve-plate tower arranged above a liquid sulfur tank. The liquid sulfur contained in the liquid sulfur tank is elevated in pressure via a pump, and enters the stripping tower from therebelow together with air. The liquid sulfur and the air are in close contact with each other via sieve pores. H2SX dissolved in the liquid sulfur decomposes into H2S, which enters a gas phase space with the air. A resulting gas mixture is fed into an incinerator or a reaction furnace.
If exhaust gases generated during liquid sulfur degassing are fed to an incinerator and discharged after incineration therein, sulfur-containing substances such as hydrogen sulfide, sulfur vapor, and the like contained therein will be converted into SO2 after combustion. As a result, the concentration of SO2 in the flue gas emissions will be increased to range from 100 to 300 mg/m3, which cannot satisfy the requirements of the new environment protection standard. On the other hand, if exhaust gasses generated during liquid sulfur degassing are fed to the reaction furnace for recycle of sulfur, the temperature in the furnace box of the reaction furnace will be reduced by about 30° C. Such a decrease of temperature in the furnace box has to be compensated for by an acid gas preheating measure, thus increasing energy consumption of the plant to a large extent, and sizes of the pipe and devices accordingly. In addition, a potential risk arises that the high concentration acid gas might return to the liquid sulfur tank.